Raise a better error message if dbt parses the same file twice due to misconfiguration

[drewbanin]

Describe the feature

If a macro is addressed twice in a dbt project, then dbt will fail to compile with:

Compilation Error
  dbt found two macros named "<macro name>" in the project "projectname".
   To fix this error, rename or remove one of the following macros:
      - dbt_modules/package/macros/my_macro.sql
      - dbt_modules/package/macros/my_macro.sql

This is confusing because there is actually on a single file and the macro is not defined twice, dbt just doesn't recognize that it has already parsed the file.

The root cause of an error like this is a misconfiguration like:

macro-paths: [macros, dbt_modules/package/macros]

Instead, dbt should raise the following error if a file is parsed twice in parsing:

Compilation Error
  The file <filepath> was parsed multiple times by dbt.
  This error happens when a path is duplicated in a dbt_project.yml configuration.
  Check your `dbt_project.yml` file path configurations and remove any
  duplicated paths to fix this error.

Describe alternatives you've considered

We could optionally allow the duplicate paths, raise a warning, and continue on. I'm amenable to that, but I don't think there's any real merit to supporting it!

Who will this benefit?

Clearer error messages are good for everyone